After a kettle and a teaspoon of instant coffee, it’s all dimishing returns 🙂
But to address the OP simply and directly, I’d agree that in-the-cup result is not going to be much better after you move from a 2k machine that offers accuracy and stability (temperature and pressure), to a 4k machine.
Beyond that, personally I place a lot of value on a 5% improvement so I’m prepared to pay a multiple of that 5% to achieve it.
That 5% approach also applies to bean selection, roasting, burr sets, baskets, puck prep, shower screens, water treatment and paper filters. A lot of five percenters can add up to make quite a difference. It’s an expensive approach and at times I feel anally retentive, so it’s not for everyone.
I value and pay more for other aspects too such as aesthetics and how much I enjoy a particular piece of kit. And in the case of the Decent, I place value on the recipes/presets that have been developed and refined by various luminaries.
All of which is to say, that whilst a 2k machine can do a stellar job and a 4k machine may not add much (if anything) in-the-cup, there may be other reasons for spending more than the 2k.
My two bobs worth and each to their own.